Merge tag 'trace-ringbuffer-v6.16' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull ring-buffer updates from Steven Rostedt:

 - Allow the persistent ring buffer to be memory mapped

   In the last merge window there was issues with the implementation of
   mapping the persistent ring buffer because it was assumed that the
   persistent memory was just physical memory without being part of the
   kernel virtual address space. But this was incorrect and the
   persistent ring buffer can be mapped the same way as the allocated
   ring buffer is mapped.

   The metadata for the persistent ring buffer is different than the
   normal ring buffer and the organization of mapping it to user space
   is a little different. Make the updates needed to the meta data to
   allow the persistent ring buffer to be mapped to user space.

 - Fix cpus_read_lock() with buffer->mutex and cpu_buffer->mapping_lock

   Mapping the ring buffer to user space uses the
   cpu_buffer->mapping_lock. The buffer->mutex can be taken when the
   mapping_lock is held, giving the locking order of:
   cpu_buffer->mapping_lock -->> buffer->mutex. But there also exists
   the ordering:

       buffer->mutex -->> cpus_read_lock()
       mm->mmap_lock -->> cpu_buffer->mapping_lock
       cpus_read_lock() -->> mm->mmap_lock

   causing a circular chain of:

       cpu_buffer->mapping_lock -> buffer->mutex -->> cpus_read_lock() -->>
          mm->mmap_lock -->> cpu_buffer->mapping_lock

   By moving the cpus_read_lock() outside the buffer->mutex where:
   cpus_read_lock() -->> buffer->mutex, breaks the deadlock chain.

 - Do not trigger WARN_ON() for commit overrun

   When the ring buffer is user space mapped and there's a "commit
   overrun" (where an interrupt preempted an event, and then added so
   many events it filled the buffer having to drop events when it hit
   the preempted event) a WARN_ON() was triggered if this was read via a
   memory mapped buffer.

   This is due to "missed events" being non zero when the reader page
   ended up with the commit page. The idea was, if the writer is on the
   reader page, there's only one page that has been written to and there
   should be no missed events.

   But if a commit overrun is done where the writer is off the commit
   page and looped around to the commit page causing missed events, it
   is possible that the reader page is the commit page with missed
   events.

   Instead of triggering a WARN_ON() when the reader page is the commit
   page with missed events, trigger it when the reader page is the
   tail_page with missed events. That's because the writer is always on
   the tail_page if an event was interrupted (which holds the commit
   event) and continues off the commit page.

 - Reset the persistent buffer if it is fully consumed

   On boot up, if the user fully consumes the last boot buffer of the
   persistent buffer, if it reboots without enabling it, there will
   still be events in the buffer which can cause confusion. Instead,
   reset the buffer when it is fully consumed, so that the data is not
   read again.

 - Clean up some goto out jumps

   There's a few cases that the code jumps to the "out:" label that
   simply returns a value. There used to be more work done at those
   labels but now that they simply return a value use a return instead
   of jumping to a label.

 - Use guard() to simplify some of the code

   Add guard() around some locking instead of jumping to a label to do
   the unlocking.

 - Use free() to simplify some of the code

   Use free(kfree) on variables that will get freed on error and use
   return_ptr() to return the variable when its not freed. There's one
   instance where free(kfree) simplifies the code on a temp variable
   that was allocated just for the function use.

* tag 'trace-ringbuffer-v6.16' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Simplify functions with __free(kfree) to free allocations
  ring-buffer: Make ring_buffer_{un}map() simpler with guard(mutex)
  ring-buffer: Simplify ring_buffer_read_page() with guard()
  ring-buffer: Simplify reset_disabled_cpu_buffer() with use of guard()
  ring-buffer: Remove jump to out label in ring_buffer_swap_cpu()
  ring-buffer: Removed unnecessary if() goto out where out is the next line
  tracing: Reset last-boot buffers when reading out all cpu buffers
  ring-buffer: Allow reserve_mem persistent ring buffers to be mmapped
  ring-buffer: Do not trigger WARN_ON() due to a commit_overrun
  ring-buffer: Move cpus_read_lock() outside of buffer->mutex

2 files changed, 134 insertions, 105 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 6859008ca34d..e24509bd0af5 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2226,7 +2226,7 @@ static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
 static struct ring_buffer_per_cpu *
 rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
 {
-	struct ring_buffer_per_cpu *cpu_buffer;
+	struct ring_buffer_per_cpu *cpu_buffer __free(kfree) = NULL;
 	struct ring_buffer_cpu_meta *meta;
 	struct buffer_page *bpage;
 	struct page *page;
@@ -2252,7 +2252,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
 	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
 			    GFP_KERNEL, cpu_to_node(cpu));
 	if (!bpage)
-		goto fail_free_buffer;
+		return NULL;
 
 	rb_check_bpage(cpu_buffer, bpage);
 
@@ -2318,13 +2318,11 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
 		rb_head_page_activate(cpu_buffer);
 	}
 
-	return cpu_buffer;
+	return_ptr(cpu_buffer);
 
  fail_free_reader:
 	free_buffer_page(cpu_buffer->reader_page);
 
- fail_free_buffer:
-	kfree(cpu_buffer);
 	return NULL;
 }
 
@@ -2359,7 +2357,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 					 unsigned long scratch_size,
 					 struct lock_class_key *key)
 {
-	struct trace_buffer *buffer;
+	struct trace_buffer *buffer __free(kfree) = NULL;
 	long nr_pages;
 	int subbuf_size;
 	int bsize;
@@ -2373,7 +2371,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		return NULL;
 
 	if (!zalloc_cpumask_var(&buffer->cpumask, GFP_KERNEL))
-		goto fail_free_buffer;
+		return NULL;
 
 	buffer->subbuf_order = order;
 	subbuf_size = (PAGE_SIZE << order);
@@ -2472,7 +2470,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 
 	mutex_init(&buffer->mutex);
 
-	return buffer;
+	return_ptr(buffer);
 
  fail_free_buffers:
 	for_each_buffer_cpu(buffer, cpu) {
@@ -2484,8 +2482,6 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
  fail_free_cpumask:
 	free_cpumask_var(buffer->cpumask);
 
- fail_free_buffer:
-	kfree(buffer);
 	return NULL;
 }
 
@@ -2849,6 +2845,12 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 	if (nr_pages < 2)
 		nr_pages = 2;
 
+	/*
+	 * Keep CPUs from coming online while resizing to synchronize
+	 * with new per CPU buffers being created.
+	 */
+	guard(cpus_read_lock)();
+
 	/* prevent another thread from changing buffer sizes */
 	mutex_lock(&buffer->mutex);
 	atomic_inc(&buffer->resizing);
@@ -2893,7 +2895,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 			cond_resched();
 		}
 
-		cpus_read_lock();
 		/*
 		 * Fire off all the required work handlers
 		 * We can't schedule on offline CPUs, but it's not necessary
@@ -2933,7 +2934,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 			cpu_buffer->nr_pages_to_update = 0;
 		}
 
-		cpus_read_unlock();
 	} else {
 		cpu_buffer = buffer->buffers[cpu_id];
 
@@ -2961,8 +2961,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 			goto out_err;
 		}
 
-		cpus_read_lock();
-
 		/* Can't run something on an offline CPU. */
 		if (!cpu_online(cpu_id))
 			rb_update_pages(cpu_buffer);
@@ -2981,7 +2979,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 		}
 
 		cpu_buffer->nr_pages_to_update = 0;
-		cpus_read_unlock();
 	}
 
  out:
@@ -4684,10 +4681,7 @@ void ring_buffer_discard_commit(struct trace_buffer *buffer,
 	RB_WARN_ON(buffer, !local_read(&cpu_buffer->committing));
 
 	rb_decrement_entry(cpu_buffer, event);
-	if (rb_try_to_discard(cpu_buffer, event))
-		goto out;
-
- out:
+	rb_try_to_discard(cpu_buffer, event);
 	rb_end_commit(cpu_buffer);
 
 	trace_recursive_unlock(cpu_buffer);
@@ -6020,6 +6014,39 @@ static void rb_clear_buffer_page(struct buffer_page *page)
 	page->read = 0;
 }
 
+/*
+ * When the buffer is memory mapped to user space, each sub buffer
+ * has a unique id that is used by the meta data to tell the user
+ * where the current reader page is.
+ *
+ * For a normal allocated ring buffer, the id is saved in the buffer page
+ * id field, and updated via this function.
+ *
+ * But for a fixed memory mapped buffer, the id is already assigned for
+ * fixed memory ording in the memory layout and can not be used. Instead
+ * the index of where the page lies in the memory layout is used.
+ *
+ * For the normal pages, set the buffer page id with the passed in @id
+ * value and return that.
+ *
+ * For fixed memory mapped pages, get the page index in the memory layout
+ * and return that as the id.
+ */
+static int rb_page_id(struct ring_buffer_per_cpu *cpu_buffer,
+		      struct buffer_page *bpage, int id)
+{
+	/*
+	 * For boot buffers, the id is the index,
+	 * otherwise, set the buffer page with this id
+	 */
+	if (cpu_buffer->ring_meta)
+		id = rb_meta_subbuf_idx(cpu_buffer->ring_meta, bpage->page);
+	else
+		bpage->id = id;
+
+	return id;
+}
+
 static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
 	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
@@ -6028,7 +6055,9 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 		return;
 
 	meta->reader.read = cpu_buffer->reader_page->read;
-	meta->reader.id = cpu_buffer->reader_page->id;
+	meta->reader.id = rb_page_id(cpu_buffer, cpu_buffer->reader_page,
+				     cpu_buffer->reader_page->id);
+
 	meta->reader.lost_events = cpu_buffer->lost_events;
 
 	meta->entries = local_read(&cpu_buffer->entries);
@@ -6098,21 +6127,16 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
 /* Must have disabled the cpu buffer then done a synchronize_rcu */
 static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	unsigned long flags;
-
-	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
 
 	if (RB_WARN_ON(cpu_buffer, local_read(&cpu_buffer->committing)))
-		goto out;
+		return;
 
 	arch_spin_lock(&cpu_buffer->lock);
 
 	rb_reset_cpu(cpu_buffer);
 
 	arch_spin_unlock(&cpu_buffer->lock);
-
- out:
-	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 }
 
 /**
@@ -6300,37 +6324,33 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
 
 	if (!cpumask_test_cpu(cpu, buffer_a->cpumask) ||
 	    !cpumask_test_cpu(cpu, buffer_b->cpumask))
-		goto out;
+		return -EINVAL;
 
 	cpu_buffer_a = buffer_a->buffers[cpu];
 	cpu_buffer_b = buffer_b->buffers[cpu];
 
 	/* It's up to the callers to not try to swap mapped buffers */
-	if (WARN_ON_ONCE(cpu_buffer_a->mapped || cpu_buffer_b->mapped)) {
-		ret = -EBUSY;
-		goto out;
-	}
+	if (WARN_ON_ONCE(cpu_buffer_a->mapped || cpu_buffer_b->mapped))
+		return -EBUSY;
 
 	/* At least make sure the two buffers are somewhat the same */
 	if (cpu_buffer_a->nr_pages != cpu_buffer_b->nr_pages)
-		goto out;
+		return -EINVAL;
 
 	if (buffer_a->subbuf_order != buffer_b->subbuf_order)
-		goto out;
-
-	ret = -EAGAIN;
+		return -EINVAL;
 
 	if (atomic_read(&buffer_a->record_disabled))
-		goto out;
+		return -EAGAIN;
 
 	if (atomic_read(&buffer_b->record_disabled))
-		goto out;
+		return -EAGAIN;
 
 	if (atomic_read(&cpu_buffer_a->record_disabled))
-		goto out;
+		return -EAGAIN;
 
 	if (atomic_read(&cpu_buffer_b->record_disabled))
-		goto out;
+		return -EAGAIN;
 
 	/*
 	 * We can't do a synchronize_rcu here because this
@@ -6367,7 +6387,6 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
 out_dec:
 	atomic_dec(&cpu_buffer_a->record_disabled);
 	atomic_dec(&cpu_buffer_b->record_disabled);
-out:
 	return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_swap_cpu);
@@ -6526,38 +6545,37 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	struct buffer_data_page *bpage;
 	struct buffer_page *reader;
 	unsigned long missed_events;
-	unsigned long flags;
 	unsigned int commit;
 	unsigned int read;
 	u64 save_timestamp;
-	int ret = -1;
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
-		goto out;
+		return -1;
 
 	/*
 	 * If len is not big enough to hold the page header, then
 	 * we can not copy anything.
 	 */
 	if (len <= BUF_PAGE_HDR_SIZE)
-		goto out;
+		return -1;
 
 	len -= BUF_PAGE_HDR_SIZE;
 
 	if (!data_page || !data_page->data)
-		goto out;
+		return -1;
+
 	if (data_page->order != buffer->subbuf_order)
-		goto out;
+		return -1;
 
 	bpage = data_page->data;
 	if (!bpage)
-		goto out;
+		return -1;
 
-	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
 
 	reader = rb_get_reader_page(cpu_buffer);
 	if (!reader)
-		goto out_unlock;
+		return -1;
 
 	event = rb_reader_event(cpu_buffer);
 
@@ -6591,7 +6609,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 		if (full &&
 		    (!read || (len < (commit - read)) ||
 		     cpu_buffer->reader_page == cpu_buffer->commit_page))
-			goto out_unlock;
+			return -1;
 
 		if (len > (commit - read))
 			len = (commit - read);
@@ -6600,7 +6618,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 		size = rb_event_ts_length(event);
 
 		if (len < size)
-			goto out_unlock;
+			return -1;
 
 		/* save the current timestamp, since the user will need it */
 		save_timestamp = cpu_buffer->read_stamp;
@@ -6658,7 +6676,6 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 		if (reader->real_end)
 			local_set(&bpage->commit, reader->real_end);
 	}
-	ret = read;
 
 	cpu_buffer->lost_events = 0;
 
@@ -6685,11 +6702,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	if (commit < buffer->subbuf_size)
 		memset(&bpage->data[commit], 0, buffer->subbuf_size - commit);
 
- out_unlock:
-	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
-
- out:
-	return ret;
+	return read;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_page);
 
@@ -6944,23 +6957,29 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
 	unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
 	struct buffer_page *first_subbuf, *subbuf;
+	int cnt = 0;
 	int id = 0;
 
-	subbuf_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
-	cpu_buffer->reader_page->id = id++;
+	id = rb_page_id(cpu_buffer, cpu_buffer->reader_page, id);
+	subbuf_ids[id++] = (unsigned long)cpu_buffer->reader_page->page;
+	cnt++;
 
 	first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
 	do {
+		id = rb_page_id(cpu_buffer, subbuf, id);
+
 		if (WARN_ON(id >= nr_subbufs))
 			break;
 
 		subbuf_ids[id] = (unsigned long)subbuf->page;
-		subbuf->id = id;
 
 		rb_inc_page(&subbuf);
 		id++;
+		cnt++;
 	} while (subbuf != first_subbuf);
 
+	WARN_ON(cnt != nr_subbufs);
+
 	/* install subbuf ID to kern VA translation */
 	cpu_buffer->subbuf_ids = subbuf_ids;
 
@@ -7052,7 +7071,7 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 {
 	unsigned long nr_subbufs, nr_pages, nr_vma_pages, pgoff = vma->vm_pgoff;
 	unsigned int subbuf_pages, subbuf_order;
-	struct page **pages;
+	struct page **pages __free(kfree) = NULL;
 	int p = 0, s = 0;
 	int err;
 
@@ -7120,10 +7139,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 		struct page *page;
 		int off = 0;
 
-		if (WARN_ON_ONCE(s >= nr_subbufs)) {
-			err = -EINVAL;
-			goto out;
-		}
+		if (WARN_ON_ONCE(s >= nr_subbufs))
+			return -EINVAL;
 
 		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
 
@@ -7138,9 +7155,6 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 
 	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
 
-out:
-	kfree(pages);
-
 	return err;
 }
 #else
@@ -7156,36 +7170,34 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
 	unsigned long flags, *subbuf_ids;
-	int err = 0;
+	int err;
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return -EINVAL;
 
 	cpu_buffer = buffer->buffers[cpu];
 
-	mutex_lock(&cpu_buffer->mapping_lock);
+	guard(mutex)(&cpu_buffer->mapping_lock);
 
 	if (cpu_buffer->user_mapped) {
 		err = __rb_map_vma(cpu_buffer, vma);
 		if (!err)
 			err = __rb_inc_dec_mapped(cpu_buffer, true);
-		mutex_unlock(&cpu_buffer->mapping_lock);
 		return err;
 	}
 
 	/* prevent another thread from changing buffer/sub-buffer sizes */
-	mutex_lock(&buffer->mutex);
+	guard(mutex)(&buffer->mutex);
 
 	err = rb_alloc_meta_page(cpu_buffer);
 	if (err)
-		goto unlock;
+		return err;
 
 	/* subbuf_ids include the reader while nr_pages does not */
 	subbuf_ids = kcalloc(cpu_buffer->nr_pages + 1, sizeof(*subbuf_ids), GFP_KERNEL);
 	if (!subbuf_ids) {
 		rb_free_meta_page(cpu_buffer);
-		err = -ENOMEM;
-		goto unlock;
+		return -ENOMEM;
 	}
 
 	atomic_inc(&cpu_buffer->resize_disabled);
@@ -7213,35 +7225,29 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 		atomic_dec(&cpu_buffer->resize_disabled);
 	}
 
-unlock:
-	mutex_unlock(&buffer->mutex);
-	mutex_unlock(&cpu_buffer->mapping_lock);
-
-	return err;
+	return 0;
 }
 
 int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
 	unsigned long flags;
-	int err = 0;
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return -EINVAL;
 
 	cpu_buffer = buffer->buffers[cpu];
 
-	mutex_lock(&cpu_buffer->mapping_lock);
+	guard(mutex)(&cpu_buffer->mapping_lock);
 
 	if (!cpu_buffer->user_mapped) {
-		err = -ENODEV;
-		goto out;
+		return -ENODEV;
 	} else if (cpu_buffer->user_mapped > 1) {
 		__rb_inc_dec_mapped(cpu_buffer, false);
-		goto out;
+		return 0;
 	}
 
-	mutex_lock(&buffer->mutex);
+	guard(mutex)(&buffer->mutex);
 	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
 
 	/* This is the last user space mapping */
@@ -7256,12 +7262,7 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
 	rb_free_meta_page(cpu_buffer);
 	atomic_dec(&cpu_buffer->resize_disabled);
 
-	mutex_unlock(&buffer->mutex);
-
-out:
-	mutex_unlock(&cpu_buffer->mapping_lock);
-
-	return err;
+	return 0;
 }
 
 int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
@@ -7302,8 +7303,8 @@ consume:
 	/* Check if any events were dropped */
 	missed_events = cpu_buffer->lost_events;
 
-	if (cpu_buffer->reader_page != cpu_buffer->commit_page) {
-		if (missed_events) {
+	if (missed_events) {
+		if (cpu_buffer->reader_page != cpu_buffer->commit_page) {
 			struct buffer_data_page *bpage = reader->page;
 			unsigned int commit;
 			/*
@@ -7324,13 +7325,23 @@ consume:
 				local_add(RB_MISSED_STORED, &bpage->commit);
 			}
 			local_add(RB_MISSED_EVENTS, &bpage->commit);
+		} else if (!WARN_ONCE(cpu_buffer->reader_page == cpu_buffer->tail_page,
+				      "Reader on commit with %ld missed events",
+				      missed_events)) {
+			/*
+			 * There shouldn't be any missed events if the tail_page
+			 * is on the reader page. But if the tail page is not on the
+			 * reader page and the commit_page is, that would mean that
+			 * there's a commit_overrun (an interrupt preempted an
+			 * addition of an event and then filled the buffer
+			 * with new events). In this case it's not an
+			 * error, but it should still be reported.
+			 *
+			 * TODO: Add missed events to the page for user space to know.
+			 */
+			pr_info("Ring buffer [%d] commit overrun lost %ld events at timestamp:%lld\n",
+				cpu, missed_events, cpu_buffer->reader_page->page->time_stamp);
 		}
-	} else {
-		/*
-		 * There really shouldn't be any missed events if the commit
-		 * is on the reader page.
-		 */
-		WARN_ON_ONCE(missed_events);
 	}
 
 	cpu_buffer->lost_events = 0;
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 465989585135..95ae7c4e5835 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6712,6 +6712,22 @@ static int tracing_wait_pipe(struct file *filp)
 	return 1;
 }
 
+static bool update_last_data_if_empty(struct trace_array *tr)
+{
+	if (!(tr->flags & TRACE_ARRAY_FL_LAST_BOOT))
+		return false;
+
+	if (!ring_buffer_empty(tr->array_buffer.buffer))
+		return false;
+
+	/*
+	 * If the buffer contains the last boot data and all per-cpu
+	 * buffers are empty, reset it from the kernel side.
+	 */
+	update_last_data(tr);
+	return true;
+}
+
 /*
  * Consumer reader.
  */
@@ -6743,6 +6759,9 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 	}
 
 waitagain:
+	if (update_last_data_if_empty(iter->tr))
+		return 0;
+
 	sret = tracing_wait_pipe(filp);
 	if (sret <= 0)
 		return sret;
@@ -8321,6 +8340,9 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
 
 	if (ret < 0) {
 		if (trace_empty(iter) && !iter->closed) {
+			if (update_last_data_if_empty(iter->tr))
+				return 0;
+
 			if ((filp->f_flags & O_NONBLOCK))
 				return -EAGAIN;
 
@@ -8662,10 +8684,6 @@ static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
 	if (iter->tr->flags & TRACE_ARRAY_FL_MEMMAP)
 		return -ENODEV;
 
-	/* Currently the boot mapped buffer is not supported for mmap */
-	if (iter->tr->flags & TRACE_ARRAY_FL_BOOT)
-		return -ENODEV;
-
 	ret = get_snapshot_map(iter->tr);
 	if (ret)
 		return ret;